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authorAndi Kleen <ak@linux.intel.com>2011-09-15 16:06:48 -0700
committerroot <root@serles.lst.de>2011-10-28 14:58:58 +0200
commitef3d0fd27e90f67e35da516dafc1482c82939a60 (patch)
treedea852eab2a52782867becffb11bce2577ed2b91 /include/linux/fs.h
parent847cc6371ba820763773e993000410d6d8d23515 (diff)
vfs: do (nearly) lockless generic_file_llseek
The i_mutex lock use of generic _file_llseek hurts. Independent processes accessing the same file synchronize over a single lock, even though they have no need for synchronization at all. Under high utilization this can cause llseek to scale very poorly on larger systems. This patch does some rethinking of the llseek locking model: First the 64bit f_pos is not necessarily atomic without locks on 32bit systems. This can already cause races with read() today. This was discussed on linux-kernel in the past and deemed acceptable. The patch does not change that. Let's look at the different seek variants: SEEK_SET: Doesn't really need any locking. If there's a race one writer wins, the other loses. For 32bit the non atomic update races against read() stay the same. Without a lock they can also happen against write() now. The read() race was deemed acceptable in past discussions, and I think if it's ok for read it's ok for write too. => Don't need a lock. SEEK_END: This behaves like SEEK_SET plus it reads the maximum size too. Reading the maximum size would have the 32bit atomic problem. But luckily we already have a way to read the maximum size without locking (i_size_read), so we can just use that instead. Without i_mutex there is no synchronization with write() anymore, however since the write() update is atomic on 64bit it just behaves like another racy SEEK_SET. On non atomic 32bit it's the same as SEEK_SET. => Don't need a lock, but need to use i_size_read() SEEK_CUR: This has a read-modify-write race window on the same file. One could argue that any application doing unsynchronized seeks on the same file is already broken. But for the sake of not adding a regression here I'm using the file->f_lock to synchronize this. Using this lock is much better than the inode mutex because it doesn't synchronize between processes. => So still need a lock, but can use a f_lock. This patch implements this new scheme in generic_file_llseek. I dropped generic_file_llseek_unlocked and changed all callers. Signed-off-by: Andi Kleen <ak@linux.intel.com> Signed-off-by: Christoph Hellwig <hch@lst.de>
Diffstat (limited to 'include/linux/fs.h')
-rw-r--r--include/linux/fs.h9
1 files changed, 6 insertions, 3 deletions
diff --git a/include/linux/fs.h b/include/linux/fs.h
index c1884e974ff4..db85196f6308 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -964,7 +964,12 @@ struct file {
#define f_dentry f_path.dentry
#define f_vfsmnt f_path.mnt
const struct file_operations *f_op;
- spinlock_t f_lock; /* f_ep_links, f_flags, no IRQ */
+
+ /*
+ * Protects f_ep_links, f_flags, f_pos vs i_size in lseek SEEK_CUR.
+ * Must not be taken from IRQ context.
+ */
+ spinlock_t f_lock;
#ifdef CONFIG_SMP
int f_sb_list_cpu;
#endif
@@ -2398,8 +2403,6 @@ file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping);
extern loff_t noop_llseek(struct file *file, loff_t offset, int origin);
extern loff_t no_llseek(struct file *file, loff_t offset, int origin);
extern loff_t generic_file_llseek(struct file *file, loff_t offset, int origin);
-extern loff_t generic_file_llseek_unlocked(struct file *file, loff_t offset,
- int origin);
extern int generic_file_open(struct inode * inode, struct file * filp);
extern int nonseekable_open(struct inode * inode, struct file * filp);