Currently, panic() and crash_kexec() can be called at the same time. For example (x86 case): CPU 0: oops_end() crash_kexec() mutex_trylock() // acquired nmi_shootdown_cpus() // stop other cpus CPU 1: panic() crash_kexec() mutex_trylock() // failed to acquire smp_send_stop() // stop other cpus infinite loop If CPU 1 calls smp_send_stop() before nmi_shootdown_cpus(), kdump fails. In another case: CPU 0: oops_end() crash_kexec() mutex_trylock() // acquired <NMI> io_check_error() panic() crash_kexec() mutex_trylock() // failed to acquire infinite loop Clearly, this is an undesirable result. To fix this problem, this patch changes crash_kexec() to exclude others by using atomic_t panicking_cpu. V2: - Use atomic_cmpxchg() instead of spin_trylock() on panic_lock to exclude concurrent accesses - Don't introduce no-lock version of crash_kexec() Signed-off-by: Hidehiro Kawai <hidehiro.kawai.ez@xxxxxxxxxxx> Cc: Eric Biederman <ebiederm@xxxxxxxxxxxx> Cc: Vivek Goyal <vgoyal@xxxxxxxxxx> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxxxx> --- kernel/kexec.c | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+) diff --git a/kernel/kexec.c b/kernel/kexec.c index a785c10..ed75a8b 100644 --- a/kernel/kexec.c +++ b/kernel/kexec.c @@ -1472,6 +1472,18 @@ void __weak crash_unmap_reserved_pages(void) void crash_kexec(struct pt_regs *regs) { + int old_cpu, this_cpu; + + /* + * `old_cpu == -1' means we are the first comer and crash_kexec() + * was called without entering panic(). + * `old_cpu == this_cpu' means crash_kexec() was called from panic(). + */ + this_cpu = raw_smp_processor_id(); + old_cpu = atomic_cmpxchg(&panic_cpu, -1, this_cpu); + if (old_cpu != -1 && old_cpu != this_cpu) + return; + /* Take the kexec_mutex here to prevent sys_kexec_load * running on one cpu from replacing the crash kernel * we are using after a panic on a different cpu. @@ -1491,6 +1503,14 @@ void crash_kexec(struct pt_regs *regs) } mutex_unlock(&kexec_mutex); } + + /* + * If we came here from panic(), we have to keep panic_cpu + * to prevent other cpus from entering panic(). Otherwise, + * resetting it so that other cpus can enter panic()/crash_kexec(). + */ + if (old_cpu == -1) + atomic_xchg(&panic_cpu, -1); } size_t crash_get_memory_size(void) -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html