In [Link] linus mentions that acquire/release makes it clear which _particular_ memory accesses are the ordered ones, and it's unlikely to make any performance difference, so it's much better to pair up the release->acquire ordering than have a "wmb->rmb" ordering. ========================================================= update pagecache folio_mark_uptodate(folio) smp_wmb() set_bit PG_uptodate === ↑↑↑ STLR ↑↑↑ === smp_store_release(&inode->i_size, i_size) folio_test_uptodate(folio) test_bit PG_uptodate smp_rmb() === ↓↓↓ LDAR ↓↓↓ === smp_load_acquire(&inode->i_size) copy_page_to_iter() ========================================================= Calling smp_store_release() in i_size_write() ensures that the data in the page and the PG_uptodate bit are updated before the isize is updated, and calling smp_load_acquire() in i_size_read ensures that it will not read a newer isize than the data in the page. Therefore, this avoids buffered read-write inconsistencies caused by Load-Load reordering. Link: https://lore.kernel.org/r/CAHk-=wifOnmeJq+sn+2s-P46zw0SFEbw9BSCGgp2c5fYPtRPGw@xxxxxxxxxxxxxx/ Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx> --- include/linux/fs.h | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) diff --git a/include/linux/fs.h b/include/linux/fs.h index 06ecccbb5bfe..077849bfe89a 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -907,7 +907,8 @@ static inline loff_t i_size_read(const struct inode *inode) preempt_enable(); return i_size; #else - return inode->i_size; + /* Pairs with smp_store_release() in i_size_write() */ + return smp_load_acquire(&inode->i_size); #endif } @@ -929,7 +930,12 @@ static inline void i_size_write(struct inode *inode, loff_t i_size) inode->i_size = i_size; preempt_enable(); #else - inode->i_size = i_size; + /* + * Pairs with smp_load_acquire() in i_size_read() to ensure + * changes related to inode size (such as page contents) are + * visible before we see the changed inode size. + */ + smp_store_release(&inode->i_size, i_size); #endif } -- 2.31.1
