motorhead/qemu
1
0
Fork 0
mirror of https://github.com/Motorhead1991/qemu.git synced 2025-08-04 08:13:54 -06:00
Code Issues Projects Releases Packages Wiki Activity Actions

tcg: consistently access cpu->tb_jmp_cache atomically

Browse source 
Some code paths can lead to atomic accesses racing with memset()
on cpu->tb_jmp_cache, which can result in torn reads/writes
and is undefined behaviour in C11.

These torn accesses are unlikely to show up as bugs, but from code
inspection they seem possible. For example, tb_phys_invalidate does:
    /* remove the TB from the hash list */
    h = tb_jmp_cache_hash_func(tb->pc);
    CPU_FOREACH(cpu) {
        if (atomic_read(&cpu->tb_jmp_cache[h]) == tb) {
            atomic_set(&cpu->tb_jmp_cache[h], NULL);
        }
    }
Here atomic_set might race with a concurrent memset (such as the
ones scheduled via "unsafe" async work, e.g. tlb_flush_page) and
therefore we might end up with a torn pointer (or who knows what,
because we are under undefined behaviour).

This patch converts parallel accesses to cpu->tb_jmp_cache to use
atomic primitives, thereby bringing these accesses back to defined
behaviour. The price to pay is to potentially execute more instructions
when clearing cpu->tb_jmp_cache, but given how infrequently they happen
and the small size of the cache, the performance impact I have measured
is within noise range when booting debian-arm.

Note that under "safe async" work (e.g. do_tb_flush) we could use memset
because no other vcpus are running. However I'm keeping these accesses
atomic as well to keep things simple and to avoid confusing analysis
tools such as ThreadSanitizer.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Message-Id: <1497486973-25845-1-git-send-email-cota@braap.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>
This commit is contained in:
Emilio G. Cota 2017-06-14 20:36:13 -04:00 • committed by Richard Henderson
parent 53f6672bcf
commit f3ced3c592
4 changed files with 25 additions and 21 deletions
Download patch file Download diff file Expand all files Collapse all files

11
include/qom/cpu.h
View file

@ -346,7 +346,7 @@ struct CPUState {
void *env_ptr; /* CPUArchState */
/* Writes protected by tb_lock, reads not thread-safe */
/* Accessed in parallel; all accesses must be atomic */
struct TranslationBlock *tb_jmp_cache[TB_JMP_CACHE_SIZE];
struct GDBRegisterState *gdb_regs;
@ -422,6 +422,15 @@ extern struct CPUTailQ cpus;
extern __thread CPUState *current_cpu;
static inline void cpu_tb_jmp_cache_clear(CPUState *cpu)
{
unsigned int i;
for (i = 0; i < TB_JMP_CACHE_SIZE; i++) {
atomic_set(&cpu->tb_jmp_cache[i], NULL);
}
}
/**
* qemu_tcg_mttcg_enabled:
* Check whether we are running MultiThread TCG or not.