__unhash_process acquires the dcache_lock while holding the tasklist_lock for
writing. This can deadlock. While trying to fix that race, I found further
races with proc_dentry. The attached patch (hopefully) fixes all these
races.
Changes:
- fs/proc/base.c assumed that p->pid is reset to 0 during exit. This is
not the case anymore. I now look at the count of the pid structure for
PIDTYPE_PID.
- pid_delete_dentry and pid_revalidate removed. The implementation didn't
work, and noone complained.
- only d_add the new proc entry if the task is still valid: d_add+d_drop is
a race, if a lookup happens between both calls.
- add a new spinlock that protects proc_dentry.
fs/exec.c | 37 +++--------------
fs/proc/base.c | 99 ++++++++++++++++++++++++++++++++--------------
include/linux/init_task.h | 1
include/linux/proc_fs.h | 2
include/linux/sched.h | 2
kernel/exit.c | 40 ++++++------------
6 files changed, 96 insertions(+), 85 deletions(-)
diff -puN fs/exec.c~tasklist_lock-dcache_lock-inversion-fix fs/exec.c
--- 25/fs/exec.c~tasklist_lock-dcache_lock-inversion-fix 2003-04-17 19:30:42.000000000 -0700
+++ 25-akpm/fs/exec.c 2003-04-17 19:30:42.000000000 -0700
@@ -528,30 +528,6 @@ static int exec_mmap(struct mm_struct *m
return 0;
}
-static struct dentry *clean_proc_dentry(struct task_struct *p)
-{
- struct dentry *proc_dentry = p->proc_dentry;
-
- if (proc_dentry) {
- spin_lock(&dcache_lock);
- if (!d_unhashed(proc_dentry)) {
- dget_locked(proc_dentry);
- __d_drop(proc_dentry);
- } else
- proc_dentry = NULL;
- spin_unlock(&dcache_lock);
- }
- return proc_dentry;
-}
-
-static inline void put_proc_dentry(struct dentry *dentry)
-{
- if (dentry) {
- shrink_dcache_parent(dentry);
- dput(dentry);
- }
-}
-
/*
* This function makes sure the current process has its own signal table,
* so that flush_signal_handlers can later reset the handlers without
@@ -659,9 +635,11 @@ static inline int de_thread(struct task_
while (leader->state != TASK_ZOMBIE)
yield();
+ task_lock(leader);
+ task_lock(current);
+ proc_dentry1 = proc_pid_unhash(current);
+ proc_dentry2 = proc_pid_unhash(leader);
write_lock_irq(&tasklist_lock);
- proc_dentry1 = clean_proc_dentry(current);
- proc_dentry2 = clean_proc_dentry(leader);
if (leader->tgid != current->tgid)
BUG();
@@ -701,9 +679,10 @@ static inline int de_thread(struct task_
state = leader->state;
write_unlock_irq(&tasklist_lock);
-
- put_proc_dentry(proc_dentry1);
- put_proc_dentry(proc_dentry2);
+ task_unlock(current);
+ task_unlock(leader);
+ proc_pid_flush(proc_dentry1);
+ proc_pid_flush(proc_dentry2);
if (state != TASK_ZOMBIE)
BUG();
diff -puN fs/proc/base.c~tasklist_lock-dcache_lock-inversion-fix fs/proc/base.c
--- 25/fs/proc/base.c~tasklist_lock-dcache_lock-inversion-fix 2003-04-17 19:30:42.000000000 -0700
+++ 25-akpm/fs/proc/base.c 2003-04-17 19:30:42.000000000 -0700
@@ -799,17 +799,11 @@ out_unlock:
/* dentry stuff */
-/*
- * Exceptional case: normally we are not allowed to unhash a busy
- * directory. In this case, however, we can do it - no aliasing problems
- * due to the way we treat inodes.
- */
-static int pid_revalidate(struct dentry * dentry, int flags)
+static int pid_alive(struct task_struct *p)
{
- if (proc_task(dentry->d_inode)->pid)
- return 1;
- d_drop(dentry);
- return 0;
+ BUG_ON(p->pids[PIDTYPE_PID].pidptr != &p->pids[PIDTYPE_PID].pid);
+
+ return atomic_read(&p->pids[PIDTYPE_PID].pid.count);
}
static int pid_fd_revalidate(struct dentry * dentry, int flags)
@@ -840,35 +834,25 @@ static int pid_fd_revalidate(struct dent
static void pid_base_iput(struct dentry *dentry, struct inode *inode)
{
struct task_struct *task = proc_task(inode);
- write_lock_irq(&tasklist_lock);
+ spin_lock(&task->proc_lock);
if (task->proc_dentry == dentry)
task->proc_dentry = NULL;
- write_unlock_irq(&tasklist_lock);
+ spin_unlock(&task->proc_lock);
iput(inode);
}
-static int pid_delete_dentry(struct dentry * dentry)
-{
- return proc_task(dentry->d_inode)->pid == 0;
-}
-
static struct dentry_operations pid_fd_dentry_operations =
{
.d_revalidate = pid_fd_revalidate,
- .d_delete = pid_delete_dentry,
};
static struct dentry_operations pid_dentry_operations =
{
- .d_revalidate = pid_revalidate,
- .d_delete = pid_delete_dentry,
};
static struct dentry_operations pid_base_dentry_operations =
{
- .d_revalidate = pid_revalidate,
.d_iput = pid_base_iput,
- .d_delete = pid_delete_dentry,
};
/* Lookups */
@@ -1093,6 +1077,60 @@ static struct inode_operations proc_self
.follow_link = proc_self_follow_link,
};
+/**
+ * proc_pid_unhash - Unhash /proc/<pid> entry from the dcache.
+ * @p: task that should be flushed.
+ *
+ * Drops the /proc/<pid> dcache entry from the hash chains.
+ *
+ * As a special exception, dropping happens even if the directory
+ * is busy. Normally we are not allowed to unhash a busy
+ * directory. In this case, however, we can do it - no aliasing problems
+ * due to the way we treat inodes. (No idea what that means - manfred)
+ *
+ * Dropping /proc/<pid> entries and detach_pid must be synchroneous,
+ * otherwise e.g. /proc/<pid>/exe might point to the wrong executable,
+ * if the pid value is immediately reused. This is enforced by
+ * - caller must acquire spin_lock(p->proc_lock)
+ * - must be called before detach_pid()
+ * - proc_pid_lookup acquires proc_lock, and checks that
+ * the target is not dead by looking at the attach count
+ * of PIDTYPE_PID.
+ */
+
+struct dentry *proc_pid_unhash(struct task_struct *p)
+{
+ struct dentry *proc_dentry;
+
+ proc_dentry = p->proc_dentry;
+ if (proc_dentry != NULL) {
+
+ spin_lock(&dcache_lock);
+ if (!d_unhashed(proc_dentry)) {
+ dget_locked(proc_dentry);
+ __d_drop(proc_dentry);
+ } else
+ proc_dentry = NULL;
+ spin_unlock(&dcache_lock);
+ }
+ return proc_dentry;
+}
+
+/**
+ * proc_pid_flush - recover memory used by stale /proc/<pid>/x entries
+ * @proc_entry: directoy to prune.
+ *
+ * Shrink the /proc directory that was used by the just killed thread.
+ */
+
+void proc_pid_flush(struct dentry *proc_dentry)
+{
+ if(proc_dentry != NULL) {
+ shrink_dcache_parent(proc_dentry);
+ dput(proc_dentry);
+ }
+}
+
/* SMP-safe */
struct dentry *proc_pid_lookup(struct inode *dir, struct dentry * dentry)
{
@@ -1141,13 +1179,16 @@ struct dentry *proc_pid_lookup(struct in
inode->i_flags|=S_IMMUTABLE;
dentry->d_op = &pid_base_dentry_operations;
- d_add(dentry, inode);
- read_lock(&tasklist_lock);
- proc_task(dentry->d_inode)->proc_dentry = dentry;
- read_unlock(&tasklist_lock);
- if (!proc_task(dentry->d_inode)->pid)
- d_drop(dentry);
- return NULL;
+
+ spin_lock(&task->proc_lock);
+ if (pid_alive(task)) {
+ task->proc_dentry = dentry;
+ d_add(dentry, inode);
+ dentry = NULL;
+ }
+ spin_unlock(&task->proc_lock);
+ if (!dentry)
+ return NULL;
out:
return ERR_PTR(-ENOENT);
}
diff -puN include/linux/init_task.h~tasklist_lock-dcache_lock-inversion-fix include/linux/init_task.h
--- 25/include/linux/init_task.h~tasklist_lock-dcache_lock-inversion-fix 2003-04-17 19:30:42.000000000 -0700
+++ 25-akpm/include/linux/init_task.h 2003-04-17 19:30:42.000000000 -0700
@@ -101,6 +101,7 @@
.blocked = {{0}}, \
.posix_timers = LIST_HEAD_INIT(tsk.posix_timers), \
.alloc_lock = SPIN_LOCK_UNLOCKED, \
+ .proc_lock = SPIN_LOCK_UNLOCKED, \
.switch_lock = SPIN_LOCK_UNLOCKED, \
.journal_info = NULL, \
}
diff -puN include/linux/proc_fs.h~tasklist_lock-dcache_lock-inversion-fix include/linux/proc_fs.h
--- 25/include/linux/proc_fs.h~tasklist_lock-dcache_lock-inversion-fix 2003-04-17 19:30:42.000000000 -0700
+++ 25-akpm/include/linux/proc_fs.h 2003-04-17 19:30:42.000000000 -0700
@@ -87,6 +87,8 @@ extern void proc_root_init(void);
extern void proc_misc_init(void);
struct dentry *proc_pid_lookup(struct inode *dir, struct dentry * dentry);
+struct dentry *proc_pid_unhash(struct task_struct *p);
+void proc_pid_flush(struct dentry *proc_dentry);
int proc_pid_readdir(struct file * filp, void * dirent, filldir_t filldir);
extern struct proc_dir_entry *create_proc_entry(const char *name, mode_t mode,
diff -puN include/linux/sched.h~tasklist_lock-dcache_lock-inversion-fix include/linux/sched.h
--- 25/include/linux/sched.h~tasklist_lock-dcache_lock-inversion-fix 2003-04-17 19:30:42.000000000 -0700
+++ 25-akpm/include/linux/sched.h 2003-04-17 19:30:42.000000000 -0700
@@ -428,6 +428,8 @@ struct task_struct {
u32 self_exec_id;
/* Protection of (de-)allocation: mm, files, fs, tty */
spinlock_t alloc_lock;
+/* Protection of proc_dentry: nesting proc_lock, dcache_lock, write_lock_irq(&tasklist_lock); */
+ spinlock_t proc_lock;
/* context-switch lock */
spinlock_t switch_lock;
diff -puN kernel/exit.c~tasklist_lock-dcache_lock-inversion-fix kernel/exit.c
--- 25/kernel/exit.c~tasklist_lock-dcache_lock-inversion-fix 2003-04-17 19:30:42.000000000 -0700
+++ 25-akpm/kernel/exit.c 2003-04-17 19:30:42.000000000 -0700
@@ -21,6 +21,7 @@
#include <linux/ptrace.h>
#include <linux/profile.h>
#include <linux/mount.h>
+#include <linux/proc_fs.h>
#include <asm/uaccess.h>
#include <asm/pgtable.h>
@@ -31,10 +32,8 @@ extern struct task_struct *child_reaper;
int getrusage(struct task_struct *, int, struct rusage *);
-static struct dentry * __unhash_process(struct task_struct *p)
+static void __unhash_process(struct task_struct *p)
{
- struct dentry *proc_dentry;
-
nr_threads--;
detach_pid(p, PIDTYPE_PID);
detach_pid(p, PIDTYPE_TGID);
@@ -46,34 +45,25 @@ static struct dentry * __unhash_process(
}
REMOVE_LINKS(p);
- proc_dentry = p->proc_dentry;
- if (unlikely(proc_dentry != NULL)) {
- spin_lock(&dcache_lock);
- if (!d_unhashed(proc_dentry)) {
- dget_locked(proc_dentry);
- __d_drop(proc_dentry);
- } else
- proc_dentry = NULL;
- spin_unlock(&dcache_lock);
- }
- return proc_dentry;
}
void release_task(struct task_struct * p)
{
- struct dentry *proc_dentry;
task_t *leader;
+ struct dentry *proc_dentry;
BUG_ON(p->state < TASK_ZOMBIE);
atomic_dec(&p->user->processes);
+ spin_lock(&p->proc_lock);
+ proc_dentry = proc_pid_unhash(p);
write_lock_irq(&tasklist_lock);
if (unlikely(p->ptrace))
__ptrace_unlink(p);
BUG_ON(!list_empty(&p->ptrace_list) || !list_empty(&p->ptrace_children));
__exit_signal(p);
__exit_sighand(p);
- proc_dentry = __unhash_process(p);
+ __unhash_process(p);
/*
* If we are the last non-leader member of the thread
@@ -92,11 +82,8 @@ void release_task(struct task_struct * p
p->parent->cnswap += p->nswap + p->cnswap;
sched_exit(p);
write_unlock_irq(&tasklist_lock);
-
- if (unlikely(proc_dentry != NULL)) {
- shrink_dcache_parent(proc_dentry);
- dput(proc_dentry);
- }
+ spin_unlock(&p->proc_lock);
+ proc_pid_flush(proc_dentry);
release_thread(p);
put_task_struct(p);
}
@@ -107,14 +94,13 @@ void unhash_process(struct task_struct *
{
struct dentry *proc_dentry;
+ spin_lock(&p->proc_lock);
+ proc_dentry = proc_pid_unhash(p);
write_lock_irq(&tasklist_lock);
- proc_dentry = __unhash_process(p);
+ __unhash_process(p);
write_unlock_irq(&tasklist_lock);
-
- if (unlikely(proc_dentry != NULL)) {
- shrink_dcache_parent(proc_dentry);
- dput(proc_dentry);
- }
+ spin_unlock(&p->proc_lock);
+ proc_pid_flush(proc_dentry);
}
/*
_