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Possible improper locking issues #7926

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ryancaicse opened this issue Oct 30, 2021 · 4 comments
Closed

Possible improper locking issues #7926

ryancaicse opened this issue Oct 30, 2021 · 4 comments
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@ryancaicse
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HI, in this function, it is possible that the lock lock (Line 71) is not released properly because the execution does not enter the while(j < num_urls)? Also, it also seems that the lock lock could be released multiple times in the while loop at Line 97.

Thanks for checking.

curl/docs/examples/smooth-gtk-thread.c

Lines 68 to 113 in 3085ccf

void *pull_one_url(void *NaN)
{
/* Stop threads from entering unless j is incremented */
pthread_mutex_lock(&lock);
while(j < num_urls) {
CURL *curl;
gchar *http;
printf("j = %d\n", j);
http =
g_strdup_printf("xoap.weather.com/weather/local/%s?cc=*&dayf=5&unit=i\n",
urls[j]);
printf("http %s", http);
curl = curl_easy_init();
if(curl) {
FILE *outfile = fopen(urls[j], "wb");
/* Set the URL and transfer type */
curl_easy_setopt(curl, CURLOPT_URL, http);
/* Write to the file */
curl_easy_setopt(curl, CURLOPT_WRITEDATA, outfile);
curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_file);
j++; /* critical line */
pthread_mutex_unlock(&lock);
curl_easy_perform(curl);
fclose(outfile);
printf("fclose\n");
curl_easy_cleanup(curl);
}
g_free(http);
/* Adds more latency, testing the mutex.*/
sleep(1);
} /* end while */
return NULL;
}

@bagder
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bagder commented Oct 30, 2021

It does indeed look very wrong. You up for writing a pull request and fix it?

@ryancaicse
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Sorry, I don't know which is the best way to fix them due to being unaware of the shared data protected by the lock. But, here is my try. Is this enough to fix this issue?

 void *pull_one_url(void *NaN) 
 { 
   /* Stop threads from entering unless j is incremented */ 
   pthread_mutex_lock(&lock); 
   while(j < num_urls) { 
     ...;
     if(curl) { 
      ...;
       pthread_mutex_unlock(&lock); 
  
       ...;
       pthread_mutex_lock(&lock); // new added
     } 
     ...;
   } /* end while */ 
    pthread_mutex_unlock(&lock); // new added
   return NULL; 
 }

bagder added a commit that referenced this issue Oct 31, 2021
@bagder
smooth-gtk-thread.c: enhance the mutex lock use
876e9da 
Reported-by: ryancaicse on github
Fixes #7926
@bagder bagder mentioned this issue Oct 31, 2021
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@bagder
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bagder commented Oct 31, 2021

I think it needs more than that. See #7931 for my proposed fix.

@ryancaicse
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OK, thanks

@bagder bagder closed this as completed in f907fae Nov 1, 2021
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smooth-gtk-thread.c: enhance the mutex lock use curl/curl
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@bagder @ryancaicse