Simplify reading a seq variable by directly using this_cpu_read API
instead of doing this_cpu_ptr and then dereferencing it.
This also avoid the below kernel BUG: which happens when
CONFIG_DEBUG_PREEMPT is enabled
BUG: using smp_processor_id() in preemptible [00000000] code: syz-fuzzer/6927
caller is ext4_mb_new_blocks+0xa4d/0x3b70 fs/ext4/mballoc.c:4711
CPU: 1 PID: 6927 Comm: syz-fuzzer Not tainted 5.7.0-next-20200602-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x18f/0x20d lib/dump_stack.c:118
check_preemption_disabled+0x20d/0x220 lib/smp_processor_id.c:48
ext4_mb_new_blocks+0xa4d/0x3b70 fs/ext4/mballoc.c:4711
ext4_ext_map_blocks+0x201b/0x33e0 fs/ext4/extents.c:4244
ext4_map_blocks+0x4cb/0x1640 fs/ext4/inode.c:626
ext4_getblk+0xad/0x520 fs/ext4/inode.c:833
ext4_bread+0x7c/0x380 fs/ext4/inode.c:883
ext4_append+0x153/0x360 fs/ext4/namei.c:67
ext4_init_new_dir fs/ext4/namei.c:2757 [inline]
ext4_mkdir+0x5e0/0xdf0 fs/ext4/namei.c:2802
vfs_mkdir+0x419/0x690 fs/namei.c:3632
do_mkdirat+0x21e/0x280 fs/namei.c:3655
do_syscall_64+0x60/0xe0 arch/x86/entry/common.c:359
entry_SYSCALL_64_after_hwframe+0x44/0xa9
Fixes: 42f56b7a4a7d ("ext4: mballoc: introduce pcpu seqcnt for freeing PA
to improve ENOSPC handling")
Suggested-by: Borislav Petkov <bp@xxxxxxxxx>
Tested-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
Signed-off-by: Ritesh Harjani <riteshh@xxxxxxxxxxxxx>
Reported-by: syzbot+82f324bb69744c5f6969@xxxxxxxxxxxxxxxxxxxxxxxxx
---
fs/ext4/mballoc.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index a9083113a8c0..c0a331e2feb0 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -4708,7 +4708,7 @@ ext4_fsblk_t ext4_mb_new_blocks(handle_t *handle,
}
ac->ac_op = EXT4_MB_HISTORY_PREALLOC;
- seq = *this_cpu_ptr(&discard_pa_seq);
+ seq = this_cpu_read(discard_pa_seq);
if (!ext4_mb_use_preallocated(ac)) {
ac->ac_op = EXT4_MB_HISTORY_ALLOC;
ext4_mb_normalize_request(ac, ar);
--
2.25.4
