From: Hugh Dickins LTP's iogen01 doio tests used to hang nicely on 32-bit SMP when /tmp was a unionfs mount of a tmpfs, i_size_read spinning forever, waiting for a lost seqcount update: fixed by taking i_lock around i_size_write when 32-bit SMP. But akpm was dissatisfied with the resulting patch: its lack of commentary, the #ifs, the nesting around i_size_read, the lack of attention to i_blocks. I promised to redo it with the general spin_lock_32bit() he proposed; but disliked the result, partly because "32bit" obscures the real constraints, which are best commented within fsstack_copy_inode_size itself. This version adds those comments, and uses sizeof comparisons which the compiler can optimize out, instead of CONFIG_SMP, CONFIG_LSF. BITS_PER_LONG. Signed-off-by: Hugh Dickins Cc: Erez Zadok Cc: Michael Halcrow Cc: Cc: Christoph Hellwig Signed-off-by: Andrew Morton --- fs/stack.c | 58 +++++++++++++++++++++++++++++++------ include/linux/fs_stack.h | 3 - 2 files changed, 50 insertions(+), 11 deletions(-) diff -puN fs/stack.c~fsstack-fsstack_copy_inode_size-locking fs/stack.c --- a/fs/stack.c~fsstack-fsstack_copy_inode_size-locking +++ a/fs/stack.c @@ -19,16 +19,56 @@ * This function cannot be inlined since i_size_{read,write} is rather * heavy-weight on 32-bit systems */ -void fsstack_copy_inode_size(struct inode *dst, const struct inode *src) +void fsstack_copy_inode_size(struct inode *dst, struct inode *src) { -#if BITS_PER_LONG == 32 && defined(CONFIG_SMP) - spin_lock(&dst->i_lock); -#endif - i_size_write(dst, i_size_read(src)); - dst->i_blocks = src->i_blocks; -#if BITS_PER_LONG == 32 && defined(CONFIG_SMP) - spin_unlock(&dst->i_lock); -#endif + loff_t i_size; + blkcnt_t i_blocks; + + /* + * i_size_read() includes its own seqlocking and protection from + * preemption (see include/linux/fs.h): we need nothing extra for + * that here, and prefer to avoid nesting locks than attempt to + * keep i_size and i_blocks in synch together. + */ + i_size = i_size_read(src); + + /* + * But if CONFIG_LSF (on 32-bit), we ought to make an effort to keep + * the two halves of i_blocks in synch despite SMP or PREEMPT - though + * stat's generic_fillattr() doesn't bother, and we won't be applying + * quotas (where i_blocks does become important) at the upper level. + * + * We don't actually know what locking is used at the lower level; but + * if it's a filesystem that supports quotas, it will be using i_lock + * as in inode_add_bytes(). tmpfs uses other locking, and its 32-bit + * is (just) able to exceed 2TB i_size with the aid of holes; but its + * i_blocks cannot carry into the upper long without almost 2TB swap - + * let's ignore that case. + */ + if (sizeof(i_blocks) > sizeof(long)) + spin_lock(&src->i_lock); + i_blocks = src->i_blocks; + if (sizeof(i_blocks) > sizeof(long)) + spin_unlock(&src->i_lock); + + /* + * If CONFIG_SMP on 32-bit, it's vital for fsstack_copy_inode_size() + * to hold some lock around i_size_write(), otherwise i_size_read() + * may spin forever (see include/linux/fs.h). We don't necessarily + * hold i_mutex when this is called, so take i_lock for that case. + * + * And if CONFIG_LSF (on 32-bit), continue our effort to keep the + * two halves of i_blocks in synch despite SMP or PREEMPT: use i_lock + * for that case too, and do both at once by combining the tests. + * + * There is none of this locking overhead in the 64-bit case. + */ + if (sizeof(i_size) > sizeof(long) || sizeof(i_blocks) > sizeof(long)) + spin_lock(&dst->i_lock); + i_size_write(dst, i_size); + dst->i_blocks = i_blocks; + if (sizeof(i_size) > sizeof(long) || sizeof(i_blocks) > sizeof(long)) + spin_unlock(&dst->i_lock); } EXPORT_SYMBOL_GPL(fsstack_copy_inode_size); diff -puN include/linux/fs_stack.h~fsstack-fsstack_copy_inode_size-locking include/linux/fs_stack.h --- a/include/linux/fs_stack.h~fsstack-fsstack_copy_inode_size-locking +++ a/include/linux/fs_stack.h @@ -21,8 +21,7 @@ /* externs for fs/stack.c */ extern void fsstack_copy_attr_all(struct inode *dest, const struct inode *src); -extern void fsstack_copy_inode_size(struct inode *dst, - const struct inode *src); +extern void fsstack_copy_inode_size(struct inode *dst, struct inode *src); /* inlines */ static inline void fsstack_copy_attr_atime(struct inode *dest, _