On Mon, Oct 07, 2019 at 12:04:16PM +0200, Marco Elver wrote:
+RCU maintainers This might be a data-race in RCU itself.
On Mon, 7 Oct 2019 at 12:01, syzbot syzbot+134336b86f728d6e55a0@syzkaller.appspotmail.com wrote:
Hello,
syzbot found the following crash on:
HEAD commit: b4bd9343 x86, kcsan: Enable KCSAN for x86 git tree: https://github.com/google/ktsan.git kcsan console output: https://syzkaller.appspot.com/x/log.txt?x=11edb20d600000 kernel config: https://syzkaller.appspot.com/x/.config?x=c0906aa620713d80 dashboard link: https://syzkaller.appspot.com/bug?extid=134336b86f728d6e55a0 compiler: gcc (GCC) 9.0.0 20181231 (experimental)
Unfortunately, I don't have any reproducer for this crash yet.
IMPORTANT: if you fix the bug, please add the following tag to the commit: Reported-by: syzbot+134336b86f728d6e55a0@syzkaller.appspotmail.com
================================================================== BUG: KCSAN: data-race in find_next_bit / rcu_report_exp_cpu_mult
write to 0xffffffff85a7f140 of 8 bytes by task 7 on cpu 0: rcu_report_exp_cpu_mult+0x4f/0xa0 kernel/rcu/tree_exp.h:244 rcu_report_exp_rdp+0x6c/0x90 kernel/rcu/tree_exp.h:254 rcu_preempt_deferred_qs_irqrestore+0x3bb/0x580 kernel/rcu/tree_plugin.h:475 rcu_read_unlock_special+0xec/0x370 kernel/rcu/tree_plugin.h:659 __rcu_read_unlock+0xcf/0xe0 kernel/rcu/tree_plugin.h:394 rcu_read_unlock include/linux/rcupdate.h:645 [inline] batadv_nc_purge_orig_hash net/batman-adv/network-coding.c:411 [inline] batadv_nc_worker+0x13a/0x390 net/batman-adv/network-coding.c:718 process_one_work+0x3d4/0x890 kernel/workqueue.c:2269 worker_thread+0xa0/0x800 kernel/workqueue.c:2415 kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352
read to 0xffffffff85a7f140 of 8 bytes by task 7251 on cpu 1: _find_next_bit lib/find_bit.c:39 [inline] find_next_bit+0x57/0xe0 lib/find_bit.c:70 sync_rcu_exp_select_node_cpus+0x28e/0x510 kernel/rcu/tree_exp.h:375 sync_rcu_exp_select_cpus+0x30c/0x590 kernel/rcu/tree_exp.h:439 rcu_exp_sel_wait_wake kernel/rcu/tree_exp.h:575 [inline] wait_rcu_exp_gp+0x25/0x40 kernel/rcu/tree_exp.h:589 process_one_work+0x3d4/0x890 kernel/workqueue.c:2269 worker_thread+0xa0/0x800 kernel/workqueue.c:2415 kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352
Reported by Kernel Concurrency Sanitizer on: CPU: 1 PID: 7251 Comm: kworker/1:4 Not tainted 5.3.0+ #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 Workqueue: rcu_gp wait_rcu_exp_gp ==================================================================
This bug is generated by a bot. It may contain errors. See https://goo.gl/tpsmEJ for more information about syzbot. syzbot engineers can be reached at syzkaller@googlegroups.com.
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And yes, ->expmask is accessed locklessly in a few places without READ_ONCE(). So does the following (untested) patch help?
Thanx, Paul
------------------------------------------------------------------------
diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h index d632cd0..bcd2a79 100644 --- a/kernel/rcu/tree_exp.h +++ b/kernel/rcu/tree_exp.h @@ -134,7 +134,7 @@ static void __maybe_unused sync_exp_reset_tree(void) rcu_for_each_node_breadth_first(rnp) { raw_spin_lock_irqsave_rcu_node(rnp, flags); WARN_ON_ONCE(rnp->expmask); - rnp->expmask = rnp->expmaskinit; + WRITE_ONCE(rnp->expmask, rnp->expmaskinit); raw_spin_unlock_irqrestore_rcu_node(rnp, flags); } } @@ -211,7 +211,7 @@ static void __rcu_report_exp_rnp(struct rcu_node *rnp, rnp = rnp->parent; raw_spin_lock_rcu_node(rnp); /* irqs already disabled */ WARN_ON_ONCE(!(rnp->expmask & mask)); - rnp->expmask &= ~mask; + WRITE_ONCE(rnp->expmask, rnp->expmask & ~mask); } }
@@ -241,7 +241,7 @@ static void rcu_report_exp_cpu_mult(struct rcu_node *rnp, raw_spin_unlock_irqrestore_rcu_node(rnp, flags); return; } - rnp->expmask &= ~mask; + WRITE_ONCE(rnp->expmask, rnp->expmask & ~mask); __rcu_report_exp_rnp(rnp, wake, flags); /* Releases rnp->lock. */ }
@@ -332,6 +332,7 @@ static bool exp_funnel_lock(unsigned long s) static void sync_rcu_exp_select_node_cpus(struct work_struct *wp) { int cpu; + unsigned long expmask_snap; unsigned long flags; unsigned long mask_ofl_test; unsigned long mask_ofl_ipi; @@ -369,10 +370,11 @@ static void sync_rcu_exp_select_node_cpus(struct work_struct *wp) */ if (rcu_preempt_has_tasks(rnp)) rnp->exp_tasks = rnp->blkd_tasks.next; + expmask_snap = rnp->expmask; raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
/* IPI the remaining CPUs for expedited quiescent state. */ - for_each_leaf_node_cpu_mask(rnp, cpu, rnp->expmask) { + for_each_leaf_node_cpu_mask(rnp, cpu, expmask_snap) { unsigned long mask = leaf_node_cpu_bit(rnp, cpu); struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
@@ -491,7 +493,7 @@ static void synchronize_sched_expedited_wait(void) struct rcu_data *rdp;
mask = leaf_node_cpu_bit(rnp, cpu); - if (!(rnp->expmask & mask)) + if (!(READ_ONCE(rnp->expmask) & mask)) continue; ndetected++; rdp = per_cpu_ptr(&rcu_data, cpu); @@ -503,7 +505,8 @@ static void synchronize_sched_expedited_wait(void) } pr_cont(" } %lu jiffies s: %lu root: %#lx/%c\n", jiffies - jiffies_start, rcu_state.expedited_sequence, - rnp_root->expmask, ".T"[!!rnp_root->exp_tasks]); + READ_ONCE(rnp_root->expmask), + ".T"[!!rnp_root->exp_tasks]); if (ndetected) { pr_err("blocking rcu_node structures:"); rcu_for_each_node_breadth_first(rnp) { @@ -513,7 +516,7 @@ static void synchronize_sched_expedited_wait(void) continue; pr_cont(" l=%u:%d-%d:%#lx/%c", rnp->level, rnp->grplo, rnp->grphi, - rnp->expmask, + READ_ONCE(rnp->expmask), ".T"[!!rnp->exp_tasks]); } pr_cont("\n"); @@ -521,7 +524,7 @@ static void synchronize_sched_expedited_wait(void) rcu_for_each_leaf_node(rnp) { for_each_leaf_node_possible_cpu(rnp, cpu) { mask = leaf_node_cpu_bit(rnp, cpu); - if (!(rnp->expmask & mask)) + if (!(READ_ONCE(rnp->expmask) & mask)) continue; dump_cpu_task(cpu); }