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fs: fs_struct rwlock to spinlock

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fs: fs_struct rwlock to spinlock

struct fs_struct.lock is an rwlock with the read-side used to protect root and
pwd members while taking references to them. Taking a reference to a path
typically requires just 2 atomic ops, so the critical section is very small.
Parallel read-side operations would have cacheline contention on the lock, the
dentry, and the vfsmount cachelines, so the rwlock is unlikely to ever give a
real parallelism increase.

Replace it with a spinlock to avoid one or two atomic operations in typical
path lookup fastpath.

Signed-off-by: Nick Piggin <npiggin@kernel.dk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Nick Piggin 2010-08-18 04:37:33 +10:00 • committed by Al Viro
parent 44672e4fbd
commit 2a4419b5b2
5 changed files with 34 additions and 34 deletions
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10
kernel/fork.c
View file

@ -752,13 +752,13 @@ static int copy_fs(unsigned long clone_flags, struct task_struct *tsk)
struct fs_struct *fs = current->fs;
if (clone_flags & CLONE_FS) {
/* tsk->fs is already what we want */
write_lock(&fs->lock);
spin_lock(&fs->lock);
if (fs->in_exec) {
write_unlock(&fs->lock);
spin_unlock(&fs->lock);
return -EAGAIN;
}
fs->users++;
write_unlock(&fs->lock);
spin_unlock(&fs->lock);
return 0;
}
tsk->fs = copy_fs_struct(fs);
@ -1676,13 +1676,13 @@ SYSCALL_DEFINE1(unshare, unsigned long, unshare_flags)
if (new_fs) {
fs = current->fs;
write_lock(&fs->lock);
spin_lock(&fs->lock);
current->fs = new_fs;
if (--fs->users)
new_fs = NULL;
else
new_fs = fs;
write_unlock(&fs->lock);
spin_unlock(&fs->lock);
}
if (new_mm) {