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Intel Mac OS X 10.15; rv:102.0) Gecko/20100101 Thunderbird/102.6.1 Subject: Re: [PATCH 3/4] accel/tcg: Handle false negative lookup in page_check_range Content-Language: en-US From: =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= To: Richard Henderson , qemu-devel@nongnu.org Cc: iii@linux.ibm.com References: <20221224151821.464455-1-richard.henderson@linaro.org> <20221224151821.464455-4-richard.henderson@linaro.org> In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=2a00:1450:4864:20::429; envelope-from=philmd@linaro.org; helo=mail-wr1-x429.google.com X-Spam_score_int: -31 X-Spam_score: -3.2 X-Spam_bar: --- X-Spam_report: (-3.2 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-1.147, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On 28/12/22 08:24, Philippe Mathieu-Daudé wrote: > On 24/12/22 16:18, Richard Henderson wrote: >> As in page_get_flags, we need to try again with the mmap >> lock held if we fail a page lookup. >> >> Signed-off-by: Richard Henderson >> --- >>   accel/tcg/user-exec.c | 39 ++++++++++++++++++++++++++++++++------- >>   1 file changed, 32 insertions(+), 7 deletions(-) >> >> diff --git a/accel/tcg/user-exec.c b/accel/tcg/user-exec.c >> index 2c5c10d2e6..c72a806203 100644 >> --- a/accel/tcg/user-exec.c >> +++ b/accel/tcg/user-exec.c >> @@ -525,6 +525,7 @@ void page_set_flags(target_ulong start, >> target_ulong end, int flags) >>   int page_check_range(target_ulong start, target_ulong len, int flags) >>   { >>       target_ulong last; >> +    int locked, ret; > > have_mmap_lock() returns a boolean, can we declare 'locked' > as such, ... > >>       if (len == 0) { >>           return 0;  /* trivial length */ >> @@ -535,42 +536,66 @@ int page_check_range(target_ulong start, >> target_ulong len, int flags) >>           return -1; /* wrap around */ >>       } >> +    locked = have_mmap_lock(); >>       while (true) { >>           PageFlagsNode *p = pageflags_find(start, last); >>           int missing; >>           if (!p) { >> -            return -1; /* entire region invalid */ >> +            if (!locked) { >> +                /* >> +                 * Lockless lookups have false negatives. >> +                 * Retry with the lock held. >> +                 */ >> +                mmap_lock(); >> +                locked = -1; > > ... skip this confusing assignation, ... > >> +                p = pageflags_find(start, last); >> +            } >> +            if (!p) { >> +                ret = -1; /* entire region invalid */ >> +                break; >> +            } >>           } >>           if (start < p->itree.start) { >> -            return -1; /* initial bytes invalid */ >> +            ret = -1; /* initial bytes invalid */ >> +            break; >>           } >>           missing = flags & ~p->flags; >>           if (missing & PAGE_READ) { >> -            return -1; /* page not readable */ >> +            ret = -1; /* page not readable */ >> +            break; >>           } >>           if (missing & PAGE_WRITE) { >>               if (!(p->flags & PAGE_WRITE_ORG)) { >> -                return -1; /* page not writable */ >> +                ret = -1; /* page not writable */ >> +                break; >>               } >>               /* Asking about writable, but has been protected: undo. */ >>               if (!page_unprotect(start, 0)) { >> -                return -1; >> +                ret = -1; >> +                break; >>               } >>               /* TODO: page_unprotect should take a range, not a >> single page. */ >>               if (last - start < TARGET_PAGE_SIZE) { >> -                return 0; /* ok */ >> +                ret = 0; /* ok */ >> +                break; >>               } >>               start += TARGET_PAGE_SIZE; >>               continue; >>           } >>           if (last <= p->itree.last) { >> -            return 0; /* ok */ >> +            ret = 0; /* ok */ >> +            break; >>           } >>           start = p->itree.last + 1; >>       } >> + >> +    if (locked < 0) { > > ... and check for !locked here? > >> +        mmap_unlock(); >> +    } >> +    return ret; >>   } Modulo using a boolean: Reviewed-by: Philippe Mathieu-Daudé