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Don't call dlsym() in our DRM master hooks

Browse Source 
Not only is it faster to cache the function pointers, calling
dlsym() inside open()/close() can lead to deadlocks when using
Vulkan Video decoding on top of the Nvidia driver.
This commit is contained in:
Cameron Gutman
2025-10-12 22:49:29 -05:00
parent ff81f74391
commit 1144dbccb3
3 changed files with 73 additions and 27 deletions
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2
app/app.pro
View File

@@ -327,7 +327,7 @@ libdrm {
linux {
message(Master hooks enabled)
SOURCES += masterhook.c masterhook_internal.c
LIBS += -ldl
LIBS += -ldl -pthread
}
}
cuda {

83
app/masterhook.c
View File

@@ -23,6 +23,7 @@
#include <errno.h>
#include <stdbool.h>
#include <sys/stat.h>
#include <pthread.h>
#include <xf86drm.h>
#include <xf86drmMode.h>
@@ -35,6 +36,33 @@
// hooks, that is definitely not trivial.
#if SDL_VERSION_ATLEAST(2, 0, 15)
// We don't include fcntl.h, so we have to define this ourselves
typedef int (*fn_open_t)(const char *pathname, int flags, ...);
// Pointers to the real implementations of these libdrm functions
static pthread_once_t s_InitDrmFunctions = PTHREAD_ONCE_INIT;
static typeof(drmModeSetCrtc)* fn_drmModeSetCrtc;
static typeof(drmModePageFlip)* fn_drmModePageFlip;
static typeof(drmModeAtomicCommit)* fn_drmModeAtomicCommit;
static void lookupRealDrmFunctions() {
fn_drmModeSetCrtc = dlsym(RTLD_NEXT, "drmModeSetCrtc");
fn_drmModePageFlip = dlsym(RTLD_NEXT, "drmModePageFlip");
fn_drmModeAtomicCommit = dlsym(RTLD_NEXT, "drmModeAtomicCommit");
}
// Pointers to the real implementations of these libc functions
static pthread_once_t s_InitLibCFunctions = PTHREAD_ONCE_INIT;
static fn_open_t *fn_open;
static fn_open_t *fn_open64;
static typeof(close) *fn_close;
static void lookupRealLibCFunctions() {
fn_open = dlsym(RTLD_NEXT, "open");
fn_open64 = dlsym(RTLD_NEXT, "open64");
fn_close = dlsym(RTLD_NEXT, "close");
}
// Qt's DRM master FD grabbed by our hook
int g_QtDrmMasterFd = -1;
struct stat g_DrmMasterStat;
@@ -59,6 +87,9 @@ int drmModeSetCrtc(int fd, uint32_t crtcId, uint32_t bufferId,
uint32_t x, uint32_t y, uint32_t *connectors, int count,
drmModeModeInfoPtr mode)
{
// Lookup the real libdrm function pointers if we haven't yet
pthread_once(&s_InitDrmFunctions, lookupRealDrmFunctions);
// Grab the first DRM Master FD that makes it in here. This will be the Qt
// EGLFS backend's DRM FD, on which we will call drmDropMaster() later.
if (g_QtDrmMasterFd == -1 && drmIsMaster(fd)) {
@@ -70,7 +101,7 @@ int drmModeSetCrtc(int fd, uint32_t crtcId, uint32_t bufferId,
}
// Call into the real thing
int err = ((typeof(drmModeSetCrtc)*)dlsym(RTLD_NEXT, __FUNCTION__))(fd, crtcId, bufferId, x, y, connectors, count, mode);
int err = fn_drmModeSetCrtc(fd, crtcId, bufferId, x, y, connectors, count, mode);
if (err == 0 && fd == g_QtDrmMasterFd) {
// Free old CRTC state (if any)
if (g_QtCrtcState) {
@@ -92,13 +123,16 @@ int drmModeSetCrtc(int fd, uint32_t crtcId, uint32_t bufferId,
// This hook will temporarily retake DRM master to allow Qt to render while SDL has a DRM FD open
int drmModePageFlip(int fd, uint32_t crtc_id, uint32_t fb_id, uint32_t flags, void *user_data)
{
// Lookup the real libdrm function pointers if we haven't yet
pthread_once(&s_InitDrmFunctions, lookupRealDrmFunctions);
// Call into the real thing
int err = ((typeof(drmModePageFlip)*)dlsym(RTLD_NEXT, __FUNCTION__))(fd, crtc_id, fb_id, flags, user_data);
int err = fn_drmModePageFlip(fd, crtc_id, fb_id, flags, user_data);
if (err == -EACCES && fd == g_QtDrmMasterFd) {
// If SDL took master from us, try to grab it back temporarily
int oldMasterFd = takeMasterFromSdlFd();
drmSetMaster(fd);
err = ((typeof(drmModePageFlip)*)dlsym(RTLD_NEXT, __FUNCTION__))(fd, crtc_id, fb_id, flags, user_data);
err = fn_drmModePageFlip(fd, crtc_id, fb_id, flags, user_data);
drmDropMaster(fd);
if (oldMasterFd != -1) {
drmSetMaster(oldMasterFd);
@@ -111,6 +145,9 @@ int drmModePageFlip(int fd, uint32_t crtc_id, uint32_t fb_id, uint32_t flags, vo
int drmModeAtomicCommit(int fd, drmModeAtomicReqPtr req,
uint32_t flags, void *user_data)
{
// Lookup the real libdrm function pointers if we haven't yet
pthread_once(&s_InitDrmFunctions, lookupRealDrmFunctions);
// Grab the first DRM Master FD that makes it in here. This will be the Qt
// EGLFS backend's DRM FD, on which we will call drmDropMaster() later.
if (g_QtDrmMasterFd == -1 && drmIsMaster(fd)) {
@@ -122,12 +159,12 @@ int drmModeAtomicCommit(int fd, drmModeAtomicReqPtr req,
}
// Call into the real thing
int err = ((typeof(drmModeAtomicCommit)*)dlsym(RTLD_NEXT, __FUNCTION__))(fd, req, flags, user_data);
int err = fn_drmModeAtomicCommit(fd, req, flags, user_data);
if (err == -EACCES && fd == g_QtDrmMasterFd) {
// If SDL took master from us, try to grab it back temporarily
int oldMasterFd = takeMasterFromSdlFd();
drmSetMaster(fd);
err = ((typeof(drmModeAtomicCommit)*)dlsym(RTLD_NEXT, __FUNCTION__))(fd, req, flags, user_data);
err = fn_drmModeAtomicCommit(fd, req, flags, user_data);
drmDropMaster(fd);
if (oldMasterFd != -1) {
drmSetMaster(oldMasterFd);
@@ -140,22 +177,28 @@ int drmModeAtomicCommit(int fd, drmModeAtomicReqPtr req,
// hook this variant of open(), since that's what SDL uses. When we see
// the open a FD for the same card as the Qt DRM master FD, we'll drop
// master on the Qt FD to allow the new FD to have master.
int openHook(const char *funcname, const char *pathname, int flags, va_list va);
int openHook(fn_open_t *real_open, typeof(close) *real_close, const char *pathname, int flags, va_list va);
int open(const char *pathname, int flags, ...)
{
// Lookup the real libc functions if we haven't yet
pthread_once(&s_InitLibCFunctions, lookupRealLibCFunctions);
va_list va;
va_start(va, flags);
int fd = openHook(__FUNCTION__, pathname, flags, va);
int fd = openHook(fn_open, fn_close, pathname, flags, va);
va_end(va);
return fd;
}
int open64(const char *pathname, int flags, ...)
{
// Lookup the real libc functions if we haven't yet
pthread_once(&s_InitLibCFunctions, lookupRealLibCFunctions);
va_list va;
va_start(va, flags);
int fd = openHook(__FUNCTION__, pathname, flags, va);
int fd = openHook(fn_open64, fn_close, pathname, flags, va);
va_end(va);
return fd;
}
@@ -164,11 +207,14 @@ int open64(const char *pathname, int flags, ...)
// after SDL closes its DRM FD.
int close(int fd)
{
// Lookup the real libc functions if we haven't yet
pthread_once(&s_InitLibCFunctions, lookupRealLibCFunctions);
// Remove this entry from the SDL FD table
bool lastSdlFd = removeSdlFd(fd);
// Call the real thing
int ret = ((typeof(close)*)dlsym(RTLD_NEXT, __FUNCTION__))(fd);
int ret = fn_close(fd);
// If we closed the last SDL FD, restore master to the Qt FD
if (ret == 0 && lastSdlFd) {
@@ -180,15 +226,16 @@ int close(int fd)
// Reset the CRTC state to how Qt configured it
if (g_QtCrtcState) {
int err = ((typeof(drmModeSetCrtc)*)dlsym(RTLD_NEXT, "drmModeSetCrtc"))(g_QtDrmMasterFd,
g_QtCrtcState->crtc_id,
g_QtCrtcState->buffer_id,
g_QtCrtcState->x,
g_QtCrtcState->y,
g_QtCrtcConnectors,
g_QtCrtcConnectorCount,
g_QtCrtcState->mode_valid ?
&g_QtCrtcState->mode : NULL);
SDL_assert(fn_drmModeSetCrtc != NULL);
int err = fn_drmModeSetCrtc(g_QtDrmMasterFd,
g_QtCrtcState->crtc_id,
g_QtCrtcState->buffer_id,
g_QtCrtcState->x,
g_QtCrtcState->y,
g_QtCrtcConnectors,
g_QtCrtcConnectorCount,
g_QtCrtcState->mode_valid ?
&g_QtCrtcState->mode : NULL);
if (err < 0) {
SDL_LogError(SDL_LOG_CATEGORY_APPLICATION,
"Failed to restore CRTC state to Qt DRM FD: %d",

15
app/masterhook_internal.c
View File

@@ -7,7 +7,6 @@
#endif
#include "SDL_compat.h"
#include <dlfcn.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
@@ -100,7 +99,7 @@ int takeMasterFromSdlFd()
}
}
int openHook(const char *funcname, const char *pathname, int flags, va_list va)
int openHook(typeof(open) *real_open, typeof(close) *real_close, const char *pathname, int flags, va_list va)
{
int fd;
mode_t mode;
@@ -108,11 +107,11 @@ int openHook(const char *funcname, const char *pathname, int flags, va_list va)
// Call the real thing to do the open operation
if (__OPEN_NEEDS_MODE(flags)) {
mode = va_arg(va, mode_t);
fd = ((typeof(open)*)dlsym(RTLD_NEXT, funcname))(pathname, flags, mode);
fd = real_open(pathname, flags, mode);
}
else {
mode = 0;
fd = ((typeof(open)*)dlsym(RTLD_NEXT, funcname))(pathname, flags);
fd = real_open(pathname, flags);
}
// If the file was successfully opened and we have a DRM master FD,
@@ -146,7 +145,7 @@ int openHook(const char *funcname, const char *pathname, int flags, va_list va)
allocatedFdIndex = getSdlFdEntryIndex(false);
if (allocatedFdIndex >= 0) {
// Close fd that we opened earlier (skipping our close() hook)
((typeof(close)*)dlsym(RTLD_NEXT, "close"))(fd);
real_close(fd);
// dup() an existing FD into the unused slot
fd = dup(g_SdlDrmMasterFds[allocatedFdIndex]);
@@ -163,16 +162,16 @@ int openHook(const char *funcname, const char *pathname, int flags, va_list va)
}
// Close fd that we opened earlier (skipping our close() hook)
((typeof(close)*)dlsym(RTLD_NEXT, "close"))(fd);
real_close(fd);
// We are not allowed to call drmSetMaster() without CAP_SYS_ADMIN,
// but since we just dropped the master, we can become master by
// simply creating a new FD. Let's do it.
if (__OPEN_NEEDS_MODE(flags)) {
fd = ((typeof(open)*)dlsym(RTLD_NEXT, funcname))(pathname, flags, mode);
fd = real_open(pathname, flags, mode);
}
else {
fd = ((typeof(open)*)dlsym(RTLD_NEXT, funcname))(pathname, flags);
fd = real_open(pathname, flags);
}
}