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fs: don't needlessly acquire f_lock

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Before 2011 there was no meaningful synchronization between
read/readdir/write/seek. Only in commit
ef3d0fd27e90 ("vfs: do (nearly) lockless generic_file_llseek")
synchronization was added for SEEK_CUR by taking f_lock around
vfs_setpos().

Then in 2014 full synchronization between read/readdir/write/seek was
added in commit 9c225f2655e3 ("vfs: atomic f_pos accesses as per POSIX")
by introducing f_pos_lock for regular files with FMODE_ATOMIC_POS and
for directories. At that point taking f_lock became unnecessary for such
files.

So only acquire f_lock for SEEK_CUR if this isn't a file that would have
acquired f_pos_lock if necessary.

Link: https://lore.kernel.org/r/20250207-daten-mahlzeit-99d2079864fb@brauner
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Christian Brauner 2025-02-07 15:10:33 +01:00
parent 1bb772565f
commit da06e3c517
No known key found for this signature in database
GPG Key ID: 91C61BC06578DCA2
3 changed files with 20 additions and 4 deletions
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10
fs/file.c
View File

@ -1182,6 +1182,16 @@ static inline bool file_needs_f_pos_lock(struct file *file)
(file_count(file) > 1 || file->f_op->iterate_shared);
}
bool file_seek_cur_needs_f_lock(struct file *file)
{
if (!(file->f_mode & FMODE_ATOMIC_POS) && !file->f_op->iterate_shared)
return false;
VFS_WARN_ON_ONCE((file_count(file) > 1) &&
!mutex_is_locked(&file->f_pos_lock));
return true;
}
struct fd fdget_pos(unsigned int fd)
{
struct fd f = fdget(fd);

1
fs/internal.h
View File

@ -338,3 +338,4 @@ static inline bool path_mounted(const struct path *path)
return path->mnt->mnt_root == path->dentry;
}
void file_f_owner_release(struct file *file);
bool file_seek_cur_needs_f_lock(struct file *file);

13
fs/read_write.c
View File

@ -169,11 +169,16 @@ generic_file_llseek_size(struct file *file, loff_t offset, int whence,
if (whence == SEEK_CUR) {
/*
* f_lock protects against read/modify/write race with
* other SEEK_CURs. Note that parallel writes and reads
* behave like SEEK_SET.
* If the file requires locking via f_pos_lock we know
* that mutual exclusion for SEEK_CUR on the same file
* is guaranteed. If the file isn't locked, we take
* f_lock to protect against f_pos races with other
* SEEK_CURs.
*/
guard(spinlock)(&file->f_lock);
if (file_seek_cur_needs_f_lock(file)) {
guard(spinlock)(&file->f_lock);
return vfs_setpos(file, file->f_pos + offset, maxsize);
}
return vfs_setpos(file, file->f_pos + offset, maxsize);
}