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vfs: dodge smp_mb in break_lease and break_deleg in the common case

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These inlines show up in the fast path (e.g., in do_dentry_open()) and
induce said full barrier regarding i_flctx access when in most cases the
pointer is NULL.

The pointer can be safely checked before issuing the barrier, dodging it
in most cases as a result.

It is plausible the consume fence would be sufficient, but I don't want
to go audit all callers regarding what they before calling here.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20240806172846.886570-1-mjguzik@gmail.com
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Mateusz Guzik 2024-08-06 19:28:46 +02:00 committed by Christian Brauner
parent 215ab0d8af
commit 087adb4f0f
1 changed files with 12 additions and 2 deletions
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14
include/linux/filelock.h
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@ -420,28 +420,38 @@ static inline int locks_lock_file_wait(struct file *filp, struct file_lock *fl)
#ifdef CONFIG_FILE_LOCKING
static inline int break_lease(struct inode *inode, unsigned int mode)
{
struct file_lock_context *flctx;
/*
* Since this check is lockless, we must ensure that any refcounts
* taken are done before checking i_flctx->flc_lease. Otherwise, we
* could end up racing with tasks trying to set a new lease on this
* file.
*/
flctx = READ_ONCE(inode->i_flctx);
if (!flctx)
return 0;
smp_mb();
if (inode->i_flctx && !list_empty_careful(&inode->i_flctx->flc_lease))
if (!list_empty_careful(&flctx->flc_lease))
return __break_lease(inode, mode, FL_LEASE);
return 0;
}
static inline int break_deleg(struct inode *inode, unsigned int mode)
{
struct file_lock_context *flctx;
/*
* Since this check is lockless, we must ensure that any refcounts
* taken are done before checking i_flctx->flc_lease. Otherwise, we
* could end up racing with tasks trying to set a new lease on this
* file.
*/
flctx = READ_ONCE(inode->i_flctx);
if (!flctx)
return 0;
smp_mb();
if (inode->i_flctx && !list_empty_careful(&inode->i_flctx->flc_lease))
if (!list_empty_careful(&flctx->flc_lease))
return __break_lease(inode, mode, FL_DELEG);
return 0;
}