On Mon, Sep 08, 2025 at 11:55:14AM -0500, Nathan Bossart wrote: > I've started preparing this for commit, and I realized that restricting > GetNamedLWLockTranche() to shmem_startup_hook is not sufficient. > EXEC_BACKEND builds will run this hook in every backend, so unless it's > guarded behind some sort of "if (!found)" condition (i.e., only run in the > postmaster), it'll still crash. I think we just need to add some extra > notes to the docs and check for !IsUnderPostmaster, as discussed upthread.
Or... what if we just moved the request array to shared memory? -- nathan
>From 5abfc98ef5dd26e2f3c822cbe51c6038fb4e37bf Mon Sep 17 00:00:00 2001 From: Nathan Bossart <[email protected]> Date: Mon, 8 Sep 2025 13:18:59 -0500 Subject: [PATCH v3 1/1] Move named LWLock tranche request array to shared memory. --- src/backend/postmaster/launch_backend.c | 3 +++ src/backend/storage/lmgr/lwlock.c | 31 +++++++++++++++++++++---- src/include/storage/lwlock.h | 4 ++++ 3 files changed, 33 insertions(+), 5 deletions(-) diff --git a/src/backend/postmaster/launch_backend.c b/src/backend/postmaster/launch_backend.c index a38979c50e4..c5ef14e1eaa 100644 --- a/src/backend/postmaster/launch_backend.c +++ b/src/backend/postmaster/launch_backend.c @@ -101,6 +101,7 @@ typedef struct struct InjectionPointsCtl *ActiveInjectionPoints; #endif int NamedLWLockTrancheRequests; + NamedLWLockTrancheRequest *NamedLWLockTrancheRequestArray; char **LWLockTrancheNames; int *LWLockCounter; LWLockPadded *MainLWLockArray; @@ -761,6 +762,7 @@ save_backend_variables(BackendParameters *param, #endif param->NamedLWLockTrancheRequests = NamedLWLockTrancheRequests; + param->NamedLWLockTrancheRequestArray = NamedLWLockTrancheRequestArray; param->LWLockTrancheNames = LWLockTrancheNames; param->LWLockCounter = LWLockCounter; param->MainLWLockArray = MainLWLockArray; @@ -1022,6 +1024,7 @@ restore_backend_variables(BackendParameters *param) #endif NamedLWLockTrancheRequests = param->NamedLWLockTrancheRequests; + NamedLWLockTrancheRequestArray = param->NamedLWLockTrancheRequestArray; LWLockTrancheNames = param->LWLockTrancheNames; LWLockCounter = param->LWLockCounter; MainLWLockArray = param->MainLWLockArray; diff --git a/src/backend/storage/lmgr/lwlock.c b/src/backend/storage/lmgr/lwlock.c index fcbac5213a5..46c82c63ca5 100644 --- a/src/backend/storage/lmgr/lwlock.c +++ b/src/backend/storage/lmgr/lwlock.c @@ -184,14 +184,13 @@ typedef struct NamedLWLockTrancheRequest int num_lwlocks; } NamedLWLockTrancheRequest; -static NamedLWLockTrancheRequest *NamedLWLockTrancheRequestArray = NULL; - /* - * NamedLWLockTrancheRequests is the valid length of the request array. This - * variable is non-static so that postmaster.c can copy them to child processes - * in EXEC_BACKEND builds. + * NamedLWLockTrancheRequests is the valid length of the request array. These + * variables are non-static so that launch_backend.c can copy them to child + * processes in EXEC_BACKEND builds. */ int NamedLWLockTrancheRequests = 0; +NamedLWLockTrancheRequest *NamedLWLockTrancheRequestArray = NULL; /* shared memory counter of registered tranches */ int *LWLockCounter = NULL; @@ -407,6 +406,14 @@ LWLockShmemSize(void) size = add_size(size, mul_size(MAX_NAMED_TRANCHES, sizeof(char *))); size = add_size(size, mul_size(MAX_NAMED_TRANCHES, NAMEDATALEN)); + /* + * Make space for named tranche requests. This is done for the benefit of + * EXEC_BACKEND builds, which otherwise wouldn't be able to call + * GetNamedLWLockTranche() outside postmaster. + */ + size = add_size(size, mul_size(NamedLWLockTrancheRequests, + sizeof(NamedLWLockTrancheRequest))); + /* Space for the LWLock array, plus room for cache line alignment. */ size = add_size(size, LWLOCK_PADDED_SIZE); size = add_size(size, mul_size(numLocks, sizeof(LWLockPadded))); @@ -443,6 +450,20 @@ CreateLWLocks(void) ptr += NAMEDATALEN; } + /* + * Move named tranche requests to shared memory. This is done for the + * benefit of EXEC_BACKEND builds, which otherwise wouldn't be able to + * call GetNamedLWLockTranche() outside postmaster. + */ + if (NamedLWLockTrancheRequests > 0) + { + memcpy(ptr, NamedLWLockTrancheRequestArray, + NamedLWLockTrancheRequests * sizeof(NamedLWLockTrancheRequest)); + pfree(NamedLWLockTrancheRequestArray); + NamedLWLockTrancheRequestArray = (NamedLWLockTrancheRequest *) ptr; + ptr += NamedLWLockTrancheRequests * sizeof(NamedLWLockTrancheRequest); + } + /* Ensure desired alignment of LWLock array */ ptr += LWLOCK_PADDED_SIZE - ((uintptr_t) ptr) % LWLOCK_PADDED_SIZE; MainLWLockArray = (LWLockPadded *) ptr; diff --git a/src/include/storage/lwlock.h b/src/include/storage/lwlock.h index 0e9cf81a4c7..8e0d0d233b4 100644 --- a/src/include/storage/lwlock.h +++ b/src/include/storage/lwlock.h @@ -73,8 +73,12 @@ typedef union LWLockPadded extern PGDLLIMPORT LWLockPadded *MainLWLockArray; +/* forward declaration of private type for use only by lwlock.c */ +typedef struct NamedLWLockTrancheRequest NamedLWLockTrancheRequest; + extern PGDLLIMPORT char **LWLockTrancheNames; extern PGDLLIMPORT int NamedLWLockTrancheRequests; +extern PGDLLIMPORT NamedLWLockTrancheRequest *NamedLWLockTrancheRequestArray; extern PGDLLIMPORT int *LWLockCounter; /* -- 2.39.5 (Apple Git-154)
